In an on-demand type ink jet printer, ink droplets are ejected from a nozzle to be deposited on a recording member, such as a paper sheet or film, for regenerating an image on the recording member. The ink jet printer has recently come into widespread use because it is amenable to reduction in size and cost. The method of ejecting ink droplets from the printer head nozzle is classified into a method of pressurizing the ink by displacement of piezoelectric elements and a method of heating and vaporizing the ink in the nozzle by heating elements for using the pressure of the generated gas bubbles. None of these methods can vary the concentration of the ink deposited on the recording member, as a principle, such that half-tone gradation cannot be expressed in terms of ink dots (pixels) as units.
As a method for basically solving the above-mentioned problem inherent in the demand type ink jet printer, the present Assignee has developed a two-liquid mixing type printer and disclosed it in Japanese Laid-Open Patent 5-201024. This two-liquid mixing type printer is designed so that ink and a dilution liquid such as a transparent solvent are mixed together at a mixing ratio related to picture data, directly before jetting the ink, to produce a diluted ink, which is immediately jetted from the nozzle so as to be deposited on the recording member. This printer is referred to hereinafter as a carrier jet printer for demarcation from the two-valued printing on-demand type ink jet printer which cannot express the half-tone gradation. With the carrier jet printer, the ink concentration can be controlled from one discharged ink droplet to another for varying the gradation from one dot on the recording member to another.
For realizing color printing of plural colors in the on-demand type ink jet printer or the above-mentioned carrier jet printer, there is a method including arraying plural printer heads discharging inks of different colors on a straight line along the main scanning direction, moving these printer heads as one along the main scanning direction and superimposing color inks at the same position on the recording paper sheet as the recording member for printing multi-color images.
As a second method, there is disclosed in Japanese Patent Publication 3-76226 such a method in which plural printer heads, each having N nozzles at an interval of K pixels in a pre-set resolution in the sub-scanning direction, are arrayed in steps with an offset of L pixels in the sub-scanning direction, where L is an integer multiple of N, and in which these printer heads are moved as one in the main scanning direction while skipping (L-1) pixels in the sub-scanning direction for printing a multicolor image by superimposing color inks at the same position on the recording paper sheet.
With the first method, since the nozzles associated with different colors are arrayed on a straight line in the main scanning direction , the inks of other colors are superimposed on the previously deposited ink which has not been dried sufficiently. The result is bleeding of the discharged color inks thus lowering the quality of the discharged color inks, and hence the quality of the image printed on the recording paper sheet. Moreover, if the printing, that is ink emission, is done in each of the forward and backward movements in the main scanning direction, the ink superposition sequence is inconveniently reversed in the forward and reverse movements of the print heads.
With the second method, the inconvenience proper to the first method can be obviated, however, the amount of offset in the sub-scanning direction of the print heads needs to be an integer number multiple of the number of nozzles N in each print head. Therefore, if the number of nozzles are increased for increasing the printing speed in currently available printer heads, four-color print heads are arrayed with an offset in the sub-scanning direction, the amounts of offsets L of the printer heads become as large as N, 2N, 3N, . . . thus increasing the size in the sub-scanning direction of the four-color print heads in their entirety. In addition, the head movement unit (carriage) for moving the print heads is also increased in size.
In view of the above inconveniences, an object of the present invention is to provide a head device which may be more compact than the conventional head device even if a large number of nozzles are annexed for high speed printing, and an ink jet printer employing such head device.